Twin boys with Fabry's disease and 6 affected relatives were described. Limb pains and retinal vessel tortuosity were present but no patient had angiokeratomata. One boy had a severe enteropathy with small and large bowel involvement which was investigated. Thin-layer chromatography showed that excesses of ceramide di- and trihexosides were excreted in the urine. Leucocyte α-galactosidase activity was measured: hemizygous males showed very low activity, while obligate and probable heterozygous females had values intermediate between those of the patients and the normal controls.